The present invention relates to a method and apparatus for measuring the applied kilovoltage of an X-ray source such as a medical X-ray machine and in particular to a method and apparatus for measuring applied kilovoltage utilizing multiple attenuation measurements through pairs of absorbers.
There is a widespread need to measure the penetrating power, as distinguished from the intensity, of X-ray sources, especially in medical and radiological equipment. Penetrating power and KV in particular are very important in the practice of medical diagnostic radiology for several reasons. The primary factor is the critical relationship between the intensity of X-ray exposure reaching the film and the KV used. For a moderal size body part, such as human head, the x-ray energy reaching the film varies approximately in proportion to the 4.5 power of the KV. This means that a 5% change in KV will lead to a 25% change in the intensity of film exposure, which will commonly lead to a degree of under or over exposure sufficient to cause a mis-diagnosis of the patients condition. This intensity to exposure conditions is due to the combination of a wide contrast range recorded on the film and the extremely subtle changes produced by many diseased states. Furthermore, the degree of image contrast obtained is primarily influenced by the KV, with different types of examinations requiring different selections of KV. For example, if a hairline fracture in the rib bone is suspected, then a relatively low KV about 60 would be used on most patients. This choice will enhance the image contrast of calcium in the bones. On the other hand, if a diseased process in the lung tissue, such as tuberculosis or cancer, is suspected then a higher KV in the range of 110 to 130 should be used to better penetrate the ribs and other bones as well as the heart. Only in this way can disease lesions which happen to lie behind the heart or ribs be seen.
In any event, it is frequently necessary to make small adjustments to the KV in order to accommodate the range of body sizes found in any patient population. Many radiology practitioners maintain technique charts listing the KV used versus body size. In this case a miscalibrated KV will produce an inadequate image which must be corrected by readjusting either the KV or other factors such as exposure time. This situation leads to unnecessary exposure of the patients. For this reason it is especially imperative that the KV calibration be uniform among the several X-ray rooms used in a radiology department. Recently, the Food and Drug Administration of the U.S. Government has recommended, and many state governments are requiring, that all medical X-ray machines be calibrated on a regular basis.
In practice, two complimentary parameters are generally used to characterize the penetrating power of an X-ray source: half value layer (HVL) and kilovolts peak (KVp). HLV is measured by determining the thickness of material (usually aluminum) necessary to reduce the reading on an air-ionization chamber to one-half the value obtained with no material. The HVL is only weakly influenced by KV.
Since applied KV will, in general, vary throughout the duration of the exposure, there are various ways of quantifying the KV used. With single phase machines, the KV applied has a sinusoidal waveform, so that the peak KV (KVp) is the common measure used. More modern X-ray machines using 3-phase current produce complex waveforms which are neither constant nor sinusoidal. In this situation a measurement of peak KV is not the most meaningful quantity. Accordingly, some sort of average or effective KV (KVe) is used.
A further desirable characteristic of any KV measuring system is that it functions accurately over a wide range of operating conditions, including variations in milliamperes (mA) of X-ray tube current and seconds of duration of exposure. Since darkening of the X-ray film is influenced by the product of mA and time, the integrated factor, mA.times.seconds=mAS, is used to quantify these factors. That is, it is desired to measure KVe accurately over an mAS range of approximately 1 to 100 mAS.
The most direct known method of measuring KV involves the direct interconnection of a resistor divider network into the high voltage system. This method, when used in conjunction with an oscilloscope, is capable of good accuracy if the waveform is simple--for example, either constant potential or purely sinusoidal. However, for highly complex waveform patterns such as seen on 3-phase machines, the estimation of KVe from the waveform is fraught with error. Thus, use of this method is limited principally to diagnosing certain problems in the X-ray circuitry, but is inconvenient for use in routine calibration. In addition, a major disadvantage of this approach is the necessity to carry bulky and complex equipment into the X-ray room and the dangers and inconveniences of breaking into the high voltage wiring.
It is thus desirable to be able to derive the KVe or KVp from measurements made directly on the beam of X-rays. Many methods have been suggested to accomplish this, all of which employ the same basic principle. The X-ray beam is filtered with a thickness of metal (or other substance of high atomic number (Z)) sufficiently to filter a large portion of the lower energy photons. The average energy of the remaining photons is then strongly correlated with, although less than, the applied KV. This method is more sensitive to KV changes if a high Z substance is used as the filter. The KV is commonly inferred from the ratio (R) of X-ray intensities passing through two thicknesses of filter, t.sub.1 and t.sub.2. The relationship between R and KV is highly non-linear so that some sort of procedure for relating R to KV must be established for any such method. Generally, it is true that increasing both thicknesses t.sub.1 and t.sub.2 will increase the sensitivity of R to KV and diminish the sensitivity of R to HVL, which is desirable. However, increasing the thickness of the filters also reduces the intensity of the X-rays passing through the filters, thereby necessitating the measurement of weaker X-rays and thus making more difficult the ability to measure exposures at low mAS or low KVe.
Use of increased thickness of absorbers also simplifies the problem of relating R to KV; if t.sub.1 and t.sub.2 are sufficiently thick then the logarithm of the detected intensity of X-rays, I, is linearly related to the thickness of the absorber used. Such a linearity is only approximate however since the curvature of the log (I) vs. (t) relationship is related to the range of photon energies present in the beam. A linear relationship is true only if the beam is homogenous, i.e., has only one photon energy. This will never be completely true for X-ray beams generated by X-ray tubes, but becomes a more accurate approximation as absorber thickness (t) is increased.
It is the primary object of the present invention to provide an improved method and apparatus for measuring the applied KV of an X-ray source.
In addition, it is an object of the present invention to provide a method and apparatus for accurately measuring the applied KV of an X-ray source over a wide dynamic operating range.
Furthermore, it is an object of the present invention to provide an accurate KV measuring device which automatically compensates for such real world factors as a non-homogenous X-ray beam, variations in detector sensitivity, long-term electronics drift and components variation.
The KV measuring device according to the present invention employs two radiation detectors which are mounted beneath a rotatable disc within which is mounted a set of nine metallic absorbers. The filter disc is rotated by an electric motor under the control of a microprocessor computer. One function of the microprocessor is to select one of three possible KVe ranges, which in the preferred embodiment are selected to be: I=40-70 KV, II=60-100 KV, III=90-150 KV. Thus, each KV range uses three filters; two are chosen to be as close to equal in thickness, t.sub.1, as possible, while the third is somewhat thicker, t.sub.2. In the preferred embodiment, t.sub.2 is approximately 30% larger than t.sub.1.
Each measurement of X-ray KVe requires that two exposures be made. In the first exposure, called the "calibration" exposure, the absorber wheel is turned so that both detectors are filtered by the two absorbers of thickness t.sub.1. The detected X-ray intensities are measured, processed, and stored in the digital memory for future use. Between the first and second exposures, the filter wheel is automatically advanced so that one detector is filtered by t.sub.1 and the second detector by t.sub.2 thicknesses, respectively. The subsequent X-ray exposure, called the "measure" exposure, provides the additional data from which KVe can be accurately calculated. Thus, each KVe is calculated on the basis for four measurements of X-ray intensity.
The purpose of the calibrate exposure is not to measure KVe but rather to establish certain calibration factors which are important in the final KVe determination, namely: the ratio of the relative intensities falling on the two detectors, the ratio of the sensitivities of the two detectors to radiation, and the ratio of the overall amplification factors of the several amplifiers and pulse shaping networks of the two channels which transfer the detector currents to the analog to digital converters. In this way, not only is the influence of a non-homogenous X-ray beam (referred to as the "heel effect") on KVe measurement eliminated, but also eliminated is any imbalance or long term drift in the electronic amplification factors. In addition, because both filters have a thickness t.sub.1 in the calibrate mode, the ratio of the readings is not influenced by the KVe of the calibrate exposure.
Furthermore, the dynamic range of the KVe measuring device according to the present invention is enhanced by eliminating low level offset voltages and currents in the first stage of amplification. This is accomplished in the preferred embodiment by a.c. coupling of the analog signals and through real time use of the microprocessor to measure and subtract these offsets. Dynamic range of the present KVe measuring device is also enhanced by the use in the preferred embodiment of a partially integrating resister-capacitor network in the feedback loop of the first amplification circuit, so that the range of voltage produced at the output of the amplifier is determined primarily by only two factors, KVe and mAS, rather than the three parameters, KVe, mA, and time.
Another design feature which enhances the useable dynamic range of the instrument is the use of only modestly thick absorbers. In the preferred embodiment, range I uses copper filters of 0.75 and 1.0 millimeter; Range II uses 1.5 and 2.0 mm of copper, and range III uses 0.5 mm of copper as well as 1.5 and 2.0 mm of tin, respectively. These choices have the advantage of being thin enough that X-ray factors as low as 5 mAS can be used at the lower end of each KVe range.
Additional objects and advantages of the present invention will become apparent from a reading of the Detailed Description of the Preferred Embodiment which makes reference to the following set of drawings of which :